Ellipsometry is an optical technique for the characterization and observation of events at an interface or film between two media and is based on exploiting the polarization transformation that occurs as a beam of polarized light is reflected from or transmitted through the interface or film. Ellipsometry has been particularly attractive because of its essential non-perturbing character (when the wavelength and intensity of the light beam are properly chosen) hence its suitability for in-situ measurements, and its remarkable sensitivity to minute interfacial effects, such as the formation of a sparsely distributed sub-monolayer of atoms or molecules.
One way in which the light wave can interact with an optical system under study is by being reflected by a surface of the optical system. This reflection causes the state of polarization to be changed abruptly. Such change can be explained using the Fresnel reflection coefficients for the two linear polarizations parallel
(p) and perpendicular (s) to the plane of incidence. Reflection ellipsometry has been recognized as an important tool for the study of surfaces and thin films. Among the many useful applications of ellipsometry are: PA1 (1) the measure of optical properties of materials and their frequency dependence (wavelength dispersion), the materials may be in liquid or solid phase, may be optically isotropic or anisotropic, and can be either in bulk or thin-film form;
(2) monitoring of phenomena on surfaces that involve either the growth of thin films starting from a submono-layer (e.g., by oxidation, deposition, absorption or diffusion of impurities), or the removal of such films (e.g., by desorption, sputtering or diffusion); and
(3) measurement of physical factors that affect the optical properties such as electric and magnetic fields, stress or temperature. When the polarization state change depends on the angle of interaction of the light beam and the sample under study, as for example with reflection from a sample, the incident light should be as collimated as possible so only a single angle of incidence is measured at one time.
It is advantageous to measure as many angles of incidence on a particular sample as possible. However, in the past this was not done frequently because it is so cumbersome to get the data by making separate successive measurements at each angle through the use of a scanning technique or with multiple ellipsometers. This problem in the art has been overcome by the Assignee's simultaneous multiple angle/multiple wavelength ellipsometer and method as disclosed in its U.S. Pat. No. 5,166,752. The ellipsometry method and the ellipsometer disclosed therein permit the simultaneous illumination of a sample at a whole range of angles of incidence from a single beam of light and permit the rapid, easy collection of a large multiplicity of data for different angles or ranges of angles within the whole range of angles without requiring scanning or the use of multiple ellipsometers. However, a drawback of this known ellipsometry method and ellipsometer is that they do not provide a means for separation of instrument error from the measured properties. This instrument error can be significant, especially where the ellipsometer is vacuum compatible. The disclosure of Assignee's U.S. Pat. No. 5,166,752 is incorporated herein by reference.
Return path or folded-path ellipsometers are, per se, known. O'Bryan, for example, devised a folded-path ellipsometer for making measurements with collimated light interacted with an optical system at a single angle of incidence, see FIG. 3.21 on page 253, and the related discussion, in Ellipsometry and Polarized Light by R. A. Azzam and N. M. Bashara, published by North-Holland Physics Publishing, 1987 edition. However, this known return path ellipsometer does not allow for simultaneous measurements at multiple angles of incidence and/or multiple wavelengths.
There is a need for an improved ellipsometry method and ellipsometer which overcome the aforementioned disadvantages and limitations of the prior art. An object of the present invention is to provide an improved ellipsometry method and ellipsometer which solve these problems and, at the same time, which offer more accurate measurements, more measurement capability and simplicity as compared with the prior art.
These and other objects of the invention are attained by the ellipsometry method of the invention which comprises operating an ellipsometer to direct polarized light so that it interacts with a sample optical system under study, and measuring the change in polarization state of light interacted with the sample optical system, the polarized light from a single beam of light being simultaneously directed to interact with the sample optical system at different angles of incidence. The change of polarization state is measured for at least one, and preferably each of a plurality of the angles of incidence. The method further includes measuring non-sample optical system ellipsometric effects of the ellipsometer and using the results thereof to correct the measured changes in polarization state to eliminate error that is introduced by the non-sample optical system ellipsometric effects.
In the disclosed embodiment, the step of measuring non-sample optical system ellipsometric effects of the ellipsometer includes operating the ellipsometer in an error correction mode during which the sample optical system under study is effectively removed from the optical path of the ellipsometer. As a result of the nature of the ellipsometry method and ellipsometer of the invention, the ellipsometric measurements made in the error correction mode and those made with the sample optical system under study, e.g. the sample, in the optical path of the ellipsometer differ only in that the measurement performed during sample measurement includes interactions with the measured sample. All aspects of the measurement which are related to imperfections in the apparatus, e.g. the non-sample optical system ellipsometric effects, are common to both measurements and are therefore subject to elimination by appropriate mathematical technique as discussed herein.
The ellipsometry method of the invention preferably involves operating a return-path ellipsometer with a polarizer arm and an analyzer arm superimposed on or sharing components with the polarizer arm and wherein the step of directing the polarized light to interact with the sample optical system at a plurality of angles of incidence includes reflecting the light from a surface of the sample optical system and re-reflecting the reflected light from the surface of the sample optical system so that it retraces its path in the arm of the ellipsometer. In this way, the light interacts twice with the surface of the sample.
The step of measuring non-sample optical system ellipsometric effects of the ellipsometer preferably includes inserting a reflector in the optical path of the beam of polarized light directed toward the sample optical system at different angles of incidence to cause the light rays at each of the plurality of angles of incidence in the beam to retrace its path in the arm of the ellipsometer without undergoing reflection and re-reflection from the surface of the optical system as in the sample measurement step. The reflector is removed from the optical path of the beam of polarized light directed toward the optical system to permit the measuring of the change in polarization state of the light interacted with the sample optical system. In one example of the invention, a thin film under study is supported on a stage of the ellipsometer in a manner to permit adjustment of the position of the film with respect to the focusing optic of the ellipsometer. This enables the relative position of the film and focusing optic to be adjusted so that preferably the focused beam of polarized light is reflected and re-reflected from the same spot on the surface of the film.
Further, in the disclosed embodiment of the method, the polarization state of the light beam is modulated during each of the measuring steps. A portion of the light which is re-reflected to the focusing optic reaches a detector array of the ellipsometer where the intensity of a multiplicity of constituent rays, each corresponding to a different angle of incidence on the sample, is measured as a function of time. This information is sufficient to determine the ellipsometric properties of the sample at each of the angles of incidence in accordance with standard practice.
The improved ellipsometer of the invention comprises means for directing polarized light so that it interacts with a sample optical system under study, and means for measuring the change in polarization state of the light interacted with the sample optical system under study, wherein the means for directing polarized light includes means for simultaneously directing polarized light from a single beam of light onto the optical system under study at different angles of incidence and wherein the means for measuring measures the change in polarization state of light interacted with the sample optical system under study for at least one, and preferably for each of a plurality, of the different angles of incidence. Further, the ellipsometer includes means for measuring non-sample optical system ellipsometric effects of the ellipsometer to allow correction of the measured changes in polarization state of the light interacted with the optical system to eliminate error introduced therein by the non-sample optical system ellipsometric effects. The means for measuring non-sample optical system ellipsometric effects includes means for operating the ellipsometer in an error correction mode as mentioned above.
More particularly, in the disclosed embodiment, the ellipsometer is a return-path ellipsometer having a polarizer arm and an analyzer arm superimposed on the polarizer arm and wherein the means for measuring the change in polarization state of light interacted with the sample optical system under study for each of the plurality of angles of incidence reflects the light from a surface of the optical system and re-reflects the reflected light from the surface of the sample optical system so that it retraces its path in the arm of the ellipsometer for detection by the detector array. In the disclosed embodiment, the means for measuring non-sample optical system ellipsometric effects includes a reflector which can be automatically inserted in a position in the optical path of the beam of polarized light directed toward the sample optical system at different angles of incidence to cause the light rays at each of the plurality of angles of incidence in the beam to retrace its path in the arm of the ellipsometer without undergoing the reflection and re-reflection from the surface of the sample optical system.
The reflector in the disclosed embodiment is a convex mirror having a center of negative curvature thereof coincident with a focal point of a focusing optic of the ellipsometer focusing the single beam of light at a single point on the sample optical system with constituent light rays forming the plurality angles of incidence with the sample optical system. A mechanism is also provided for moving the reflector into and out of the position in the optical path. The position is proximate the optical system under study in the disclosed example. These and other objects, advantages and features of the present invention will become more apparent from the following detailed description of several embodiments of the invention taken with the accompanying drawings.
The method and ellipsometer of the invention are not limited to multi-angle ellipsometry but have applicability to return path ellipsometry and return path ellipsometers for allowing correcting for non-sample optical system ellipsometric effects such as that induced by a vacuum-window where the ellipsometer is vacuum compatible.